The wireless transmission device in a wireless communication system is provided with an amplifier which amplifies power of a transmission signal. In general, the wireless transmission device allows the amplifier to operate in the vicinity of a saturation region of the amplifier for improving power efficiency of the amplifier. However, when the amplifier is allowed to operate in the vicinity of the saturation region, non-linear distortion increases. Therefore, in order to inhibit the non-linear distortion and reduce an ACLR (adjacent channel leakage ratio) the wireless transmission device is provided with a distortion compensation device which compensates the non-linear distortion.
One of distortion compensation systems used in the distortion compensation device is a “pre-distortion (hereinafter, sometimes also referred to as PD) system”. A PD distortion compensation device multiplies a distortion compensation coefficient having a reverse characteristic, of the non-linear distortion of the amplifier by a transmission baseband signal before being input to the amplifier in advance, thereby increasing linearity of an output of the amplifier to inhibit the distortion of the output of the amplifier. The signal obtained by multiplying the transmission baseband signal by the distortion compensation coefficient is sometimes also referred to as a “pre-distortion signal (PD signal)”. Therefore, the PD signal is the signal distorted in advance according to the reverse characteristic of the non-linear distortion of the amplifier before being input, to the amplifier.
For example, there is the PD distortion compensation device including a look-up table (hereinafter, sometimes also referred to as “LUT”) in which a plurality of distortion compensation coefficients is stored, the device which indicates an address according to the power of the transmission baseband signal to the LUT to read the distortion compensation coefficient from the LUT. The distortion compensation coefficient stored in the LUT is sequentially updated such that an error between the transmission baseband signal as a reference signal and a signal output from the amplifier to be fed back (hereinafter, sometimes also referred to as a “feedback signal”) is minimum.
Related-art examples are described, for example, in Japanese Laid-open Patent Publication No. 2011-199428.
In the PD distortion compensation device using the LUT, processes in which there is the access to the LUT roughly include following three processes. That is to say, a first process is the process in which the LUT is referred to and the distortion compensation coefficient to be multiplied by the transmission baseband signal is read from the LUT (hereinafter, sometimes also referred to as “reference time reading”). A second process is the process in which the distortion compensation coefficient, the distortion compensation coefficient being an updating target, is read from the LUT for updating the distortion compensation coefficient of the LUT (hereinafter, sometimes also referred to as “update time reading”). A third process is the process in which an adjusted distortion compensation coefficient is written in the LUT (hereinafter, sometimes also referred to as “update time writing”).
Herein, the LUT of the distortion compensation device is often realized by using a dual port memory such as a dual port RAM (random access memory). The access to the LUT is performed by the above-described first to third three processes while the number of ports of the dual port memory is two. Therefore, it is important to efficiently access the LUT by appropriately dividing the access to the LUT occurring in the three processes into the two ports.
Herein, the distortion compensation coefficient is multiplied by all the transmission baseband signals, so that the reference time reading is performed in real time. That is to say, it is not preferable to intermittently perform the reference time reading. On the other hand, it is sufficient to update the distortion compensation coefficient such that desired distortion compensation accuracy can be maintained, so that the update time reading and the update time writing are not necessary performed in real time. That is to say, it is allowed that the update time reading and the update time writing are intermittently performed.
Therefore, the technology illustrated in FIG. 1 is suggested. FIG. 1 is a view for illustrating a problem. In FIG. 1, “R” represents the reference time reading, “WR” represents the update time reading, and “WW” represents the update time writing. The same applies to the following drawings. That is to say, out of two ports of the LUT being a port 1 and a port 2, the reference time reading is always performed by using the port 1. In contrast, the update time reading and the update time writing are performed by using the port 2 of the LUT. That is to say, a plurality of times of update time reading is collectively performed from the port 2, a plurality of distortion compensation coefficients read from the LUT is temporarily stored in a memory, a plurality of distortion compensation coefficients stored in the memory is adjusted, and a plurality of adjusted distortion compensation coefficients is collectively written in the LUT. For example, when the distortion compensation coefficients for 1000 samples of transmission baseband signals are updated, in the port 2, the update time reading of the 1000 samples is collectively performed, and thereafter, the update time writing of 1000 samples is collectively performed. In this manner, by collectively and sequentially performing a plurality of times of update time reading and collectively and sequentially performing a plurality of times of update time writing in a time-division manner, it becomes possible to avoid competition between the update time reading and the update time writing in one port and perform the update time reading and the update time writing by using one port.
However, in the technology to collectively and sequentially perform a plurality of times of update time reading and collectively and sequentially perform a plurality of times of update time writing in a time-division manner, the memory which stores a plurality of distortion compensation coefficients read from the LUT is provided in the distortion compensation device, so that a circuit scale of the distortion compensation device becomes large. When the circuit scale of the distortion compensation device becomes large, a circuit scale of the wireless transmission device provided with the distortion compensation device also becomes large.